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Radiosity, Indirect Lighting and Global Illumination are rendering terms which refer to the process of accounting for the reflection of direct lights off of illuminated surfaces.

Unlike direct illumination algorithms, which tend to simulate light reflecting only once off each surface, Radiosity simulates the many reflections of light around a scene, generally resulting in softer, more natural shadows.

Algorithm

Most Radiosity renderers simulate actual light by taking the brightes light source and treating the light it adds to each surface as an additional light. After each bright light has been processed, then the process is continued by trating the illuminated surfaces as lights as well.

The process can go on for ever, because each light which is applied illimunates other surfaces - which now need to be recalculated.

Samples

This is a living room model in SketchUp. It is primarily illuminated by the sun coming through the sky light, although there is a lamp turned on in the corner.

This is a Ray Trace rendering of the living room. Only the direct light from the sun through the skylights is seen, and the lamp appears as well